Have you ever considered the possibility that we are the first to begin inhabiting the material universe? Assuming that it is God's intention to fill the universe with human inhabitants, and that the universe would be inhabited from a starting point and expanded from there, (admittedly, all assumptions), then, someone would have to be the first planet of inhabitants.
And, logically, (again, assuming the Creator keeps to Himself His long range plans), it could be expected that some on that first planet might wonder what all the rest of the universe was for. It might also be logical, considering that humans are capable of rebelling, to have the rebellion happen while there was still just one planet of us. Settle those problems there, then proceed to expand to other planets.
So, in a sense, your statement that if other planets were inhabited, that this would raise problems with the concept of Christianity would essentially be correct. And in line with that, the problems are being solved at the correct time - when there is still only one planet of humans.
There might be a hint in the Bible that this is God's purpose. In Gen 15:5 and 22:17 God promises Abraham that his offspring would be as numerous as the stars and the sand. Gal 4:29 says that it is those having faith in Christ who become Abraham's offspring
So, one might think God was exaggerating a bit with Abraham. But if you take God at His word, His promise to Abraham would, eventually, require expansion beyond planet earth.
Sorry if this idea has already been mentioned.